Always check the spurs. Young roosters should have small nubby spurs and the color of the spur will match the leg color. Older roosters will of course have bigger spurs and they will darken with age. Even if the spurs are trimmed, look for diameter of the nub and its color. I always check ear lobes too. Combs and wattles can be big even on younger roosters, but I've noticed the ear lobes will typically grow according to age. You can also check the beak for wear and legs for a bright clean color. Just take a look at the over all bird you just see the youth in them. An old bird looks like an old bird.